The present invention relates to a continuous vacuum refining method for recycling scraps of metals, in particular aluminum alloy scraps, into wrought materials or the like by eliminating impurity metallic elements, such as Zn and Mg that have high vapor pressures, and gas components such as hydrogen from the molten liquid of the metals, and an apparatus utilizable for these purposes.
As a method for effectively utilizing resources, recycling of various kinds of scrap materials has become an important problem. However, impurity elements should be often eliminated before recycling. For example, Zn must be eliminated for recycling of steel sheets plated with Zn. Harmful elements such as Bi, Pb and Cd should be eliminated for recycling copper scraps.
In the case of aluminum materials for automobiles, for example as a tube for flowing a refrigerant into an aluminum radiator, a three-layer composite material in which a filler alloy is used on one surface of a core alloy, and a sacrificial anode material is clad on the other surface of the core is used. Further a material formed by flame-spraying Zn on the surface is used as the aluminum materials of the tube or the like. Since separation of composite layers themselves from the scrap materials generated in the manufacturing process of the tubes is difficult, the composite materials themselves as low-grade scrapes are used as raw materials of cast aluminum and deoxidizing materials for molten steel. In addition, since disassembling of the scrap of the aluminum radiator (as waste disposals) takes much labor, it is used as a low-grade scrap without applying any additional processing. Most of other aluminum scraps are also recycled by similar method as the radiator.
However, most of alloys prescribed in JIS-3000 mainly used as the core material of the tube and alloys prescribed in JIS-4000 mainly used as the filler alloys contain Zn and Mg, and alloys prescribed in JIS-7072 used as a sacrificial anode material contain Zn. The aluminum alloys or aluminum alloy composite materials containing Zn and Mg as principal ingredients are also used for fins and header plates as the other constitution materials of the radiator.
Since a large quantity of Zn and Mg are contained as elements of an alloy in the radiator made of aluminum, recycling of the scraps of the tube and radiator as the raw materials of the core alloy, filler alloy and fin will become possible when a refining technology for eliminating these alloy elements is developed. As a result, reduction of material costs as well as consumption of resources will be realized.
A vacuum processing method has been known in the art as a method for eliminating impurity elements in the molten metal, and various refining method taking advantage of this technology has been proposed.
As shown in FIG. 7 in the method disclosed, for example, in JP-A-06-145831 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application), the molten liquid (33) introduced into a airtight type refining furnace (32) from a holding furnace (31) is treated in a vacuum atmosphere directly or with stirring, thereby allowing the impurity elements such as Zn and Mg contained in the molten liquid in layer proportion to evaporate. These evaporated impurity elements are recovered by re-melting in the same vessel after solidification by cooling.
However, since the conventional method is a so-called batch method by which a large quantity of the molten liquid is treated using a large scale furnace, it involved the following problems: (1) a wide installation space is required; (2) a large capacity evacuation facility is necessary for evacuating the space in the large size furnace, besides requiring a large size stirring device for effective mechanical stirring of the large quantity of the molten liquid, thereby forcing much facility cost; (3) the evacuation apparatus becomes larger for maintaining a required degree of vacuum when a stirring effect is achieved by blowing an inert gas into the molten liquid; and (4) much time and large amount of resources are lost for pressure reduction and restoration processes.
The inventors of the present invention have developed, through intensive studies for solving the foregoing problems, a continuous vacuum refining method and an apparatus thereof that can exhibit an excellent refining ability with low cost, and have disclosed the results in JP-A-11-256251. This method and apparatus shown in FIGS. 8 and 9 are quite remarkable in that a molten liquid (41) melted under an atmospheric pressure is introduced into an evacuated vessel (43), and a refined molten liquid is continuously discharged from the evacuated vessel while stirring with a stirrer (44) and recovered into a refined molten liquid recovery chamber (50) placed under an atmospheric pressure outside of the evacuated chamber. In FIGS. 8, 9A and 9B, the reference numeral (42) denotes a conduit pipe, the reference numeral (45) denotes a molten liquid feed pipe, the reference numeral (46) denotes an impurity vapor recovery part, the reference numeral (47) denotes an evacuation apparatus, and the reference numeral (49) denotes a refined molten liquid.
However, the elevation of the molten liquid column arising from the differential pressure reaches about 5 m particularly in the aluminum or magnesium alloys having a small specific gravity, since the molten liquid refined by the method described above is withdrawn under an atmospheric pressure. Accordingly, the method still involves some improvements regarding construction expenses (including foundation construction costs) and maintenance of facilities.
Other and further features and advantages of the invention will appear more fully from the following description, take in connection with the accompanying drawings.